This invention relates to air conditioning systems for cooling the air in and pressurizing an operator's compartment of mobile equipment, particularly large, off-the-road vehicles such as crawler tractors, earth movers, etc.
As a consequence of the recent legislation and the desire to improve working conditions, the demand for apparatus designed to effectively control the atmosphere within the operator's compartment of mobile industrial equipment has become increasingly common.
Attempts have been made to condition air in various vehicles by various means. The well-known system utilizing refrigeration apparatus including, among other things, a motor-driven compressor, an evaporator, a condenser and a motor-driven fan, interrelated to withdraw heat from the air in the area to be cooled and exhaust said heat into the atmosphere by the heat-exchanger principle which employs a fluid refrigerant is, broadly considered, a well-developed art; and it has been often applied to mobile equipment.
In the prevalent commercial applications relating to mobile equipment, it is customary to power the refrigerant compressing unit directly from the prime-mover which is employed to motivate the vehicle. However, in such cases as the speed of the vehicle engine varies, the compressor is driven at corresponding speeds thus effecting fluctuations in the resultant cooling function of the refrigeration systems. The resulting fluctuations are not significant in most cases for on-the-road vehicles where normal use is at higher engine speeds. While cooling in excess of requirements occurs during high engine RPM the resultant atmospheric effect is adequately controlled by utilizing a temperature-responsive clutch mechanism in the power transmission means to the compressor. However, when the engine RPM is low, undercooling results and nothing can be done by the vehicle operator to increase its cooling capacity except to increase the engine RPM. In off-the-road vehicles, the operator's responsibilities of making power and gear ratio changes while manipulating the steering and auxiliary equipment, as for example, a dozer blade's angle and elevation, would be increased to an impossible level were he additionally required to correlate engine RPM to cooling requirements.
To minimize the occurrences of undercooling, it is customary to provide a compressor having a compromise refrigerant delivery capability adequate at moderately low engine RPM and then increase the "idle" ENGINE RPM somewhat to minimize the ENGINE RPM segment between its idle speed and the minimum compressor speed, adequate to properly cool the operator's compartment during periods of maximum possible ambient temperatures. This practice requires greater compressor delivery capability than required where the compressor is continuously driven at its optimum design speed. Thus, the initial cost of an over-size compressor must be borne by the vehicle purchaser.
Furthermore, since the prevalent mobile equipment air conditioner is driven by a variable speed power source; it would compound the cooling capacity inadequacies to permit the air circulation to vary directly proportional to the air conditioner's undercooling. In other words, when the compressor's capacity to provide heat-exchanging refrigerant diminishes, and the air circulation diminishes proportionately, the rate of temperature increase in the operator's compartment will accelerate proportionately from external heat sources. To overcome this undesirable effect, it is customary to drive the air circulating fans from the vehicle's electrical system, utilizing the battery to maintain a constant power source even though the electrical supply from the generator or alternator may vary with the engine RPM.
As before, the cost to the vehicle buyer is increased because of the necessity for adding or increasing the capacity of an electrical system as well as the time involvement and expense of maintaining a secondary power source and its attendant apparatus.
Another problem with air conditioners used on off-the-road vehicles is caused by the dust raised by the vehicle. This dust has a tendency to be sucked into the unit by the evaporator fan, thus accumulating within the unit and clogging the filters.